Air filters have been used to remove pollen, dust, and the like in the air. Air filters use filter media, which are often made of nonwoven fabrics. Nonwoven fabrics are produced by various processes, and among them melt-blowing process is widely employed for the production of filter media for air filters, battery separators, and the like. Melt-blowing process is generally a process in which hot-air blows a thermoplastic polymer extruded from a spinneret to form fine filaments and the filaments self-bond together into a fiber web. As compared with other production processes of nonwoven fabrics (e.g. spun-bonding), melt-blowing process has the advantages of requiring no complicated operations and being capable of easily producing fine fibers several micrometers to several tens micrometers in diameter.
Air filters are required to have performance such as capabilities of collecting a large amount of fine dust (high collection efficiency) and of exhibiting low resistance to airflow passing through the air filters (low pressure drop). For providing a filter medium exhibiting a high collection efficiency, a nonwoven fabric formed from fibers having a small fineness is suitable. However, the structure of such a nonwoven fabric is likely to easily collapse, which increases the fiber density resulting in an increased pressure drop. For providing a filter medium exhibiting a low pressure drop, a nonwoven fabric formed from fibers having a large fineness is suitable. However, the fiber surface area of such a nonwoven fabric decreases resulting in a decreased collection efficiency. Thus, a high collection efficiency is incompatible with a low pressure drop.
For solving the above problems, an attempt has been made to simultaneously satisfy a high collection efficiency and a low pressure drop by utilizing not only physical properties but also electrostatic properties through electret treatment of a nonwoven fabric.
For example, a method for producing an electret nonwoven fabric has been proposed, the method comprising bringing a nonwoven fabric in contact with a ground electrode, and applying a high voltage to the nonwoven fabric with a non-contact voltage applying electrode, while moving the ground electrode that carries the nonwoven fabric, thereby performing continuous electret treatment (see Patent Literature 1). In this method, electrons are injected into the nonwoven fabric, ions are transferred, and dipoles are oriented. As a result, polarization is induced and a charge is imparted to the nonwoven fabric.
Another proposed method comprises adding an additive to fibers to be formed into a nonwoven fabric, thereby producing a nonwoven fabric exhibiting a high collection efficiency and a low pressure drop. For example, a heat resistant electret material has been proposed, the heat resistant electret material being made of a polymer material containing at least one stabilizer selected from a hindered amine stabilizer, a nitrogen-containing hindered phenol stabilizer, a metallic salt-hindered phenol stabilizer and a phenol stabilizer and having a trapped charge of 2.0×10−10 coulomb/cm2 or more as determined by thermally stimulated depolarization currents at a temperature of 100° C. or higher (see Patent Literature 2).
Still other proposed methods include a production method of a meltblown nonwoven fabric comprising mixed filaments made of different polymers with different characteristics (Patent Literature 3) and a production method of a nonwoven fabric in which fine fibers and coarse fibers are mixed so as to reduce the collapse of the nonwoven fabric structure and thereby to suppress an increase in pressure drop (see Patent Literature 4 and 5). However, Patent Literature 3 describes no specific combination of fiber diameters suitable for filters. Patent Literature 4 describes that a mixed-fiber nonwoven fabric in which fibers having different fiber diameters of 1 to 10 μm are appropriately mixed and dispersed is suitable for filters. However, a mixed-fiber nonwoven fabric comprising only fibers having a fiber diameter of less than 10 μm failed to achieve a sufficient effect of suppressing an increase in pressure drop especially when used as a filter medium for air filters. Patent Literature 5 discloses a method in which fibers having fiber diameters of more than 10 μm are mixed. However, the method requires a high mass per unit area in order to achieve a sufficient collection efficiency and thus the production cost for a filter medium will increase.
As described above, there has been no method capable of achieving good balance between the collection efficiency and the pressure drop and capable of producing an inexpensive filter medium.